1. Field of the Invention
This invention relates generally to oriented polymeric containers and processes for manufacturing the same; specifically, to oriented multilayer containers having at least one layer of polypropylene (“PP”) and a layer of a barrier material providing a barrier to migration of oxygen, carbon dioxide, fragrance or flavor.
2. Background
Many products desirable of being stored in plastic containers have required a barrier to control migration of carbon dioxide, oxygen, fragrance, flavor, etc. in order to maintain product freshness. Such products included, by way of example only, certain carbonated beverages, fruit juices, beers, sauces, ketchups, jams, jellies and dry foods such as instant coffees and spices. Most commercially acceptable transparent or semi-transparent containers that provided carbon dioxide and oxygen migration control were constructed of at least one layer comprising a polyester such as polyethylene terephthalate (“PET”) and a barrier layer comprising ethylene vinyl alcohol copolymer (“EVOH”), nylon or other known barrier material. The polyester layer deterred migration of moisture, although poorly so when compared to other polymers such as PP, while the barrier layer provided an excellent barrier to migration of carbon dioxide, oxygen, etc.
When biaxially oriented, PET has long been known to be stronger and have lower haze values than PP. PET has also been known to provide a better barrier to oxygen and carbon dioxide migration than PP. Containers have, nonetheless long been constructed of PP because PP provided a better barrier to moisture migration than PET. For example, PP has been used to construct extrusion blow molded multilayer containers having one or more PP layers and a barrier layer to provide a PP container with oxygen or carbon dioxide migration control. Such containers were only afforded the monoaxial orientation inherent in the extrusion blow molding process. Clarity of these bottles suffered accordingly. Monolayer biaxially oriented PP containers constructed by injection stretch blow molding or reheat stretch blow molding processes have also been employed to produce low haze oriented PP (“OPP”) structures.
Historically, PP has been significantly cheaper to purchase as a raw material than has PET. PP has been known to better withstand the high temperatures associated with hot-fill products than has PET. PP has been known to have a lower glass transition temperature, is semi-crystalline and crystallizes at a lower temperature than PET. Additionally, PP has been known to have less built in strain than PET.
Beneficially, the melt temperature of most commercial grade PP has been known to be substantially lower than that of PET, bringing the PP melt temperature closer to that of EVOH. Unfortunately traditional PP did not readily bond to most commercially feasible barrier materials. Failure to bond a barrier layer to an adjacent structural layer (such as of PET or PP) was made obvious to the naked eye due to reflection or refraction of light and detracted from the clarity and aesthetics of a resulting structure. Known PP containers with barrier protection therefore employed a discrete layer of an adhesive agent between a barrier layer and each adjacent PP layer to assure interlayer adhesion. This discrete layer of adhesive agent significantly reduced the clarity (i.e. increased the haze value) of the container. Moreover, known PP containers having a barrier layer were restricted to extrusion blow molding and the mono-axial orientation afforded thereby. The mono-axial orientation afforded by extrusion blow molding left the PP with significantly higher haze values than its PET counterpart.
Having been burdened with the discrete layer of adhesive agent and being afforded only the monoaxial orientation of extrusion blow molding, known PP containers with barrier protection suffered from high haze values. Known PP containers with barrier protection have haze values of approximately 40-70%. Despite the advantages of PP, PET has, therefore, long been the material of choice for barrier containers when low haze was desired.